The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.
Known spark-ignition (SI) engines introduce an air/fuel mixture into each cylinder that is compressed in a compression stroke and ignited by a spark plug. Known compression-ignition (CI) engines inject pressurized fuel into a combustion cylinder near top dead center (TDC) of the compression stroke that ignites upon injection. Combustion for both SI engines and CI engines involves premixed or diffusion flames controlled by fluid mechanics.
SI engines may operate in a variety of different combustion modes, including a homogeneous SI combustion mode and a stratified-charge SI combustion mode. SI engines may be configured to operate in a homogeneous-charge compression-ignition (HCCI) combustion mode, also referred to as controlled auto-ignition combustion, under predetermined speed/load operating conditions. The HCCI combustion mode includes a distributed, flameless, auto-ignition combustion process that is controlled by oxidation chemistry. An engine operating in the HCCI combustion mode has a cylinder charge that is preferably homogeneous in composition, temperature, and residual exhaust gases at intake valve closing time. HCCI combustion is a distributed kinetically-controlled combustion process with the engine operating at a dilute air/fuel mixture, i.e., lean of a stoichiometric air/fuel point, with relatively low peak combustion temperatures, resulting in low NOx emissions. The homogeneous air/fuel mixture minimizes occurrences of rich in-cylinder combustion zones that form smoke and particulate emissions.
Engine airflow may be controlled by selectively adjusting position of the throttle valve and openings and closings of intake and exhaust valves. On engine systems so equipped, openings and closings of the intake and exhaust valves may be adjusted using a variable valve actuation system that includes variable cam phasing and a selectable multi-step valve lift, e.g., multiple-step cam lobes that provide two or more valve lift positions. In contrast to the throttle position change, the change in valve position of the multi-step valve lift mechanism is a discrete step change.
When an engine operates in a HCCI combustion mode, the engine operates at a lean or stoichiometric air/fuel ratio with the throttle wide open to minimize engine pumping losses. When the engine operates in the SI combustion mode, the engine operates in stoichiometric air/fuel ratio, with the throttle valve controlled over a range of positions from 0% to 100% of the wide-open position to control intake airflow to achieve the stoichiometric air/fuel ratio.
In an engine configured to operate in SI and HCCI combustion modes, transitioning between combustion modes may be complex. The engine control module must coordinate activations of multiple devices in order to provide a desired air/fuel ratio for the different modes. During a transition between a HCCI combustion mode and SI combustion mode, valve lift switching occurs nearly instantaneously, while adjustments to cam phasers and pressures in the manifold have slower dynamics. Until the desired air/fuel ratio is achieved, incomplete combustion and misfire may occur, leading to torque disturbances.
Timing of auto-ignition combustion during engine operation in the HCCI combustion mode is affected by cylinder charge gas temperature before and during compression prior to ignition and by mixture composition of a cylinder charge. A desired auto-ignition timing associated with maximum efficiency is achieved by accounting for all influencing parameters affecting the cylinder charge gas temperature.
Known engines operating in auto-ignition combustion modes account for operating conditions using calibration tables as part of an overall engine control scheme executed in an engine control module. Known HCCI engine control schemes include calibrations for controlling engine parameters based on a limited number of input parameters including, e.g., engine load, engine speed and engine coolant temperature. Measured output parameters are used to control (among others) the amount of hot residuals (via variable cam phasing) and the amount of cold residuals (via exhaust gas recirculation rate) and therefore control in-cylinder gas temperature.
Known control systems use feedback control algorithms to compensate for effects of environmental and ambient parameters on ignition timing and air/fuel ratio. Complex multidimensional calibration tables may be used to account for all influencing parameters.
Known engines operating in HCCI combustion mode at mid-range load conditions may add fuel late in a combustion cycle to provide additional hydrocarbons in an exhaust gas feedstream to generate ammonia for NOx reduction, thus consuming fuel without a corresponding torque benefit.
Known aftertreatment systems for engines configured for operating in HCCI combustion modes may have active injection systems for dosing urea or other reductants into the exhaust gas feedstream for selective catalyst reduction.
During engine refiring subsequent to a fuel cutoff event, known engines operate at stoichiometry or rich of stoichiometry to consume oxygen stored in a three-way catalytic converter and prevent NOx breakthrough associated with lean engine operation, thus consuming fuel without a corresponding torque benefit.